It is known that there are devices for controlling the presence of information about images in video signals applicable to a video receiver with a display screen. These known devices usually comprise a memory in which reference image information is recorded and corresponds to the images the receiver is able to receive. When the video signals are received by the receiver, the information concerning reference images is compared with the image information present in the video signals received by the receiver so as to trigger an alarm when agreement is not obtained.
In these devices, the information recorded concerns all the scanning lines of the screen for the two interlaced frames, which usually allows an image to be displayed.
These known types of devices are extremely costly and are not very high-performing since, first of all, they require recording of the information of the reference images for all the screen scanning lines and for the two frames required for restoring each image; these devices also require a line by line comparison of all the information concerning images in the received video signals, together with all the reference image information. As a result, these devices require memories having large capacities and involving extremely long processing time, thus rendering these devices as being expensive and unusable in real time.
Moreover, they did not allow for any real time detection of any absence of the video signal, especially when the level of amplitude of the video signals transmitted to a receiver is lower than a receiving threshold.